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The X3 (2009) developed a high pitched ringing or high frequency sound. It does not appear to be coming from the engine but rather from under the car. It comes and goes but is particularly evident at idle? Any thoughts on what could be?

If it's present at idle put it in park and get down on the ground and see if you can narrow down the area the noise is coming from. Maybe have somone sit in the drivers seat to rev the engine a bit. Report back what you find.